1. Field of the Invention
The invention relates to a voltage step up regulator for producing a frequency control voltage for a frequency synthesizer in a radio transmitter from a low operating voltage.
2. Description of the Related Art
In a traditional frequency synthesizer formed by a phase-locked loop, frequency is usually produced by oscillators controlled by capacitance diodes. Conventional circuits include the Collpits and its various modifications. To save space and costs, the oscillator is operated at a so-called final frequency while the transmitter oscillator is frequency-modulated through a control voltage. It is desirable that this kind of oscillator can be adjusted within a frequency range as wide as possible with one and the same control voltage.
To widen the adjustable frequency range without increasing the control voltage range, the ratio of the change of frequency to the change of the control voltage has to be increased.
The problem is that noise occurring in the control voltage also modulates the carrier wave to a relatively greater degree than previously. This, in turn, results in a reduction in the signal-to-noise ratio of the radio transmission. Noise occurring in the control voltage may be caused by several different sources and it is difficult to filter. With low-frequency noise, the filtration is especially difficult to carry out.
Another way of widening the adjustable frequency range of the frequency synthesizer is to increase the control voltage range. However, portable radio devices have to use accumulators as a source of power, the typical accumulator voltage being about 12 V. Losses caused by various filter and regulator circuits further decrease the applicable dc voltage. However, a control voltage of about 12 V is insufficient for the control of the frequency synthesizer within a wide frequency range.
In prior art solutions, attempts have been made to increase such low operating voltages by circuits realized by conventional switched mode power supplies. A switched mode circuit usually operates at a frequency between 10 to 100 kHz. False signals, i.e. low-frequency noise, occuring in non-linear switched mode circuits are easily propagated in the circuits of the radio device, being summed to the intermediate and low-frequency signals.
Noise also occurs in the frequency control voltage of the frequency synthesizer, which further impairs the signal-to-noise ratio of the system. In an attempt to reduce the noise caused by the switched more circuit, the circuit has been realized by a separate hybrid, it has been positioned in a metallic casing, and the operating frequency of the circuit has been selected suitably with a frequency range between the radio channels such that it has not been taken into account in the specifications of the system.
Accordingly, the prior art switched mode circuits impair the properties of the radio device and take quite a lot a space, which is more and more inconvenient with ever decreasing device sizes.